When diagnosing or designing rotating equipment—such as industrial pumps, compressors, or turbines—vibration is the primary threat to reliability.
When diagnosing or designing rotating equipment—such as industrial pumps, compressors, or turbines—vibration is the primary threat to reliability. If your system vibrates excessively, choosing the wrong structural simulation pathway leads to useless data and wasted development hours. The confusion usually boils down to choosing between two foundational dynamic methods: Modal Analysis and Harmonic Response Analysis.
Modal Analysis: What It Tells You (and What It Doesn't)
Modal analysis is an intrinsic, unforced simulation. It solves the raw eigenvalues of the system to extract:
- Natural Frequencies: The rates at which the structure naturally wants to vibrate.
- Mode Shapes: The structural deformation shapes corresponding to each natural frequency.
What modal analysis cannot predict: It does not calculate actual stress values or displacement millimeters because there is no physical force applied.
Harmonic Response Analysis: What It Adds
Harmonic response analysis is a forced dynamic simulation. It applies a continuous, sinusoidal load across a spectrum of frequencies to evaluate how the system behaves under steady-state operating conditions. This matches the physics of rotating machinery perfectly, where mass imbalances create cyclic forces acting at specific operating speeds (orders).
Comparison Table: When to Use Which
What is the difference between harmonic response analysis and modal analysis?
Direct Answer: Modal analysis calculates a structure's inherent natural frequencies and deformation shapes without any external loads. Harmonic response analysis calculates the actual physical displacement, velocity, acceleration, and stress fields caused by a continuous, frequency-dependent cyclic force (like a rotating shaft imbalance) acting over time.
The Critical Role of Damping
In modal analysis, damping is largely irrelevant. In harmonic response, if your operating speed hits a structural natural frequency, the resulting peak stress is governed entirely by your damping ratio ($\zeta$). Guessing this value wrongly renders the model inaccurate.
Practical Example: Pump Skid Assessment
A vibration consultant troubleshooting a vibrating pump skid uses a two-step process:
- Modal First: Discovering that the frame has a natural mode at 29 Hz.
- Harmonic Second: Simulating the pump operating from 0 to 40 Hz with a 100-gram imbalance to check if the 29 Hz peak generates stresses above the steel fatigue limit.